We show that Skeletonema marinoi suppresses chain formation in response to copepod cues. The presence of
three different copepod species (Acartia tonsa, Centropages hamatus, or Temora longicornis) significantly reduced
chain length. Furthermore, chain length was significantly reduced when S. marinoi was exposed to chemical cues
from caged A. tonsa without physical contact with the responding cells. The reductions in chain length
significantly reduced copepod grazing; grazing rates on chains (four cells or more) were several times higher
compared to that of single cells. This suggests that chain length plasticity is a means for S. marinoi to reduce
copepod grazing. In contrast, chain length was not suppressed in cultures exposed to the microzooplankton grazer
Gyrodinium dominans. Size-selective predation may have played a key role in the evolution of chain formation and
chain length plasticity in diatoms